Field of the Invention
This invention relates to an apparatus for detecting degradation of an arrester, for example, of zinc oxide type, and more particularly to an apparatus for easily and precisely detecting degradation of an arrester connected to a high tension a.c. power transmission line.
Discussion of Background
Arresters employing resistors having an excellent non-linear characteristic, e.g., zinc oxide type arresters employing zinc oxide elements, have so far been widely used for protecting various items of electric equipment used in electric power systems from lightning discharges or the like. For the arresters of this type, detecting degradation by measuring the leakage current passing through its resistive component is in general an important question in the maintenance of the arresters.
An apparatus to measure such resistive-component leakage current of the arrester is disclosed for example, in Japanese Patent Publication No. 60-7356/1985. However, since this apparatus can only measure the fundamental frequency portion of the resistive-component leakage current of the arrester, there is a problem of accuracy in its measurement.
Another degradation detecting apparatus is described in Japanese Patent Laid-Open No. 55-32439/1980, which is shown in FIG. 6 of the present disclosure. This apparatus extracts the total leakage current flowing through an arrester 2 connected between a power transmission line 1 and ground using a detector 3 constituted of a resistor, current transformer, or the like, and the same is converted into a signal Io corresponding thereto. This signal Io is applied to a degradation detecting apparatus 4. Meanwhile, the zinc oxide element constituting the arrester 2 can be described by means of the equivalent circuit shown in FIG. 7, in which the total leakage current Io of the arrester is given by a vector sum of a capacitive component (Ic) which does not vary with degradation of the arrester and the resistive component (Ir) which varies with the degradation. Since normally Ic&gt;Ir, "peak value of Io=peak value of Ic" holds as shown in FIG. 8. Making use of this fact, the degradation detecting apparatus 4 automatically derives a signal for the capacitive-component leakage current Ic from the above mentioned signal for the entire leakage current Io, by means of a Schmitt circuit 5 converting an input into a rectangular wave, a low-pass filter 6 converting an input into a sine wave, a phase shifter circuit 7 compensating for a phase delay in an input, a peak value detector circuit 8 detecting the peak value of an input, and an automatic gain control (AGC) circuit 9, and further, the signal for the capacitive-component leakage current Ic is added, as a cancelling wave signal, to the above mentioned signal for the total leakage current Io by means of a differential amplifier 10, and thereby a signal for the resistive-component leakage current Ir is provided.
The above described arrester degradation detecting apparatus has an advantage over the previously mentioned apparatus described in Japanese Patent Publication No. 60-7356 in that the waveform of the resistive-component leakage current can be faithfully reproduced, and is not limited to the basic frequency component, but there is a problem in its phase shifter circuit 7. That is, while the phase shifter circuit 7 is used for compensating for the delay of the phase of the signal for the total leakage current Io from the phase of the signal for the capacitive-component current Ic, the mentioned phase difference is, in fact, varied by the level of the signal for the resistive-component leakage current Ir, and from this fact is produced a measurement error making precise measurement impossible. If it is desired to solve such a problem, a potential transformer PT or the like may be used to extract a voltage signal, for example, as described in Japanese Patent Publication No. 57-10651/1982, but if this approach is followed, the apparatus becomes larger in size and also becomes complex, and further, measurement itself becomes impossible in the case where the voltage signal cannot be extracted.
Besides, in the measurement with the apparatus of FIG. 6, if the phase adjustment should be made each time to improve accuracy of the measurement, it would become necessary to observe waveforms using an oscilloscope or the like, and this would be unfit for the initial purpose to make simple measurement in the field possible.